This work focuses on the normal development of the cerebral cortex using neuroanatomical and behavioral techniques to study developing axonal connections in mice and rats. We have concentrated on the cortico-spinal projection and the role played by collateral elimination in the development of cortical connections. Experiments using homotopic and heterotopic transplants of rat cerebral cortex during development have identified position within the tangential plane of the host cortex as a critical factor in determining which of the initially extended projections cortical neurons will maintain. However, our observations on forelimb placing behaviors in such preparations suggest that some early specification of cortical regional locale may occur. Experiments combining both behavioral observations and anterograde axonal tracing have explored the role of any lesion-induced aberrant ipsilateral corticospinal projections in the sparing of sensorimotor behaviors by the homotopic transplants. The results indicate that aberrant ipsilateral corticospinal projection fibers are present if a neonatal rostral cortical cavity is filled with either a homotopic or a heterotopic E17 transplant. Results from the behavioral tests suggest that these fibers contribute to vibrissae tactile placing, but not to forelimb tactile behavior seen after E17 homotopic transplants. Thus forelimb placing behavior in animals with neonatal lesions and transplants is influenced by both the transplanted tissue, and by an aberrant corticospinal projection from the unlesioned hemisphere. Additional experiments using anterograde axonal tracers to label corticospinal axons have begun to revealed the time course and distribution of these aberrant ipsilateral corticospinal projections in rats with neonatal damage to one rostral cortical hemisphere. These studies indicate that this aberrant projection may rise through the maintenance and elaboration of a small ipsilateral component of the corticospinal pi projection which is normally present only transiently during development.